Displays, electronic, optoelectronic, microwave, RF, and electrical devices are typically mounted, or printed, on substrates that provide support, arrange electrical power and signal supply, and act to remove heat from the device. Flexible electronic substrates are built on a flexible material base, which normally is a polymer film or metal foil. Flexible electronic substrates (FES) are also named flexible circuits or, flexible PCBs, flex prints or flexi-circuits.
Polymer films are the most common materials used for building FESs and are typically made of a polyester (for example polyethylene terephthalate (PET)), polyimide (PI), polyethylene napthalate (PEN), Polyetherimide (PEI), or one of various fluoropolymers (FEP) and copolymers.
Flexible electronic substrates (FESs) based on polymer films are limited to application in supporting electronic devices that generate low specific heat energy, for example flexible displays, organic light-emitting diodes (OLEDs), key boards, or photovoltaic devices. Such limitations in the application of FESs are partially due to the low thermal conductivity of polymer films (lower than 1 W/mK), which does not allow dissipation of heat from electronic devices. Limitations are also partially due to low thermal, structural, and dimensional stability of polymer films.
The maximum processing temperature of many polymer materials is lower than the temperatures required for thin film transistor (TFT) fabrication, which may be greater than 300° C.
For applications that are temperature critical, such as photovoltaic devices or thermoelectric devices operating at high temperatures, or flexible displays or light sources like OLEDs in which generated heat has an adverse effect of longevity, light efficiency, colour stability, and reliability of the device, it is beneficial to provide a FES with higher thermal conductivity.
For RF and microwave applications it may be beneficial that a FES comprises a dielectric material that has a high dielectric constant as well as having a ground metal layer or metal shielding layer.
In order to provide higher processing and working temperatures, and to improve thermal properties, flexible substrates may be built on metal foils such as Steel, Titanium (Ti), or Aluminium (Al) foils, which provide high temperature stability and heat dissipation.
As illustrated by the figures in Table 1, of the above mentioned metals used as a base for FESs Al has significantly higher thermal conductivity (above 150 W/mK) and is, therefore, beneficial for thermal management.
TABLE 1Thermal conductivity of materials used forflexible electronic substratesMaterialThermal conductivity (W/mK)Aluminium150-250Steel15-25Titanium 5-23Polyimide0.4-0.8Polyester0.15-0.24
To form a FES having a metal foil base, a dielectric layer is applied on the metal foil surface in order to insulate it from an electrical circuit. To maintain thermal advantages of metal based FESs it is advantageous to have a non-organic dielectric layer. A dielectric layer may be applied to a metal surface by physical-vapour deposition (PVD), or chemical-vapour deposition (CVD), by jet printing, or by anodising. The presence of natural oxide layer on the surface of some metals (for example Al) can create adhesion problems for traditional deposition coatings or printing techniques. An anodising process does not present the same problem because an anodised coating is formed by electrochemical oxidation of the substrate itself.
U.S. Pat. No. 4,015,987 describes an anodised, non-flexible, Al substrate for use as an insulated metal substrate for electronic applications. The process described in U.S. Pat. No. 4,015,987 includes anodising an aluminium substrate and laminating copper foil to the anodised substrate. Photo resist, etching and plating steps follow. Anodised Al substrates have not found broad application as FESs because of the inherent low flexibility of the anodised layer. Anodised layers also have low thermal stability. Both of these deficiencies lead to formation of micro cracks in the anodic layer, which compromise the dielectric strength of the layer.
It is an aim of the invention to provide a flexible electronic substrate having improved properties.